1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film using laser light (hereafter referred to as laser annealing), and to a laser apparatus for performing laser annealing (an apparatus containing a laser and an optical system for guiding the laser light output from the laser to a processing piece. In addition, the present invention relates to a semiconductor device formed by using that type of laser annealing method, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
The development of thin film transistors (hereafter referred to as TFTs) has been advancing in recent years, and TFTs using a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film have been in the spotlight. In particular, the TFTs are used as elements forming a driver circuit for controlling a pixel, or an element which switches the pixel, in a liquid crystal display device (liquid crystal display) or an EL (electroluminescence) display device (EL display).
A technique of crystallizing an amorphous silicon film into a polysilicon film is generally used as a means of obtaining the polysilicon film. In particular, recently a method of crystallizing the amorphous silicon film using laser light has been gathering attention. A means of obtaining a crystalline semiconductor film by crystallizing an amorphous semiconductor film using laser light is referred to as laser crystallization throughout this specification.
Instantaneous heat treatment of the semiconductor film is possible with laser crystallization, and laser crystallization is an effective technique as a means of annealing the semiconductor film formed on a substrate having low heat resistance, such as a glass substrate or a plastic substrate. Furthermore, the throughput is remarkably high compared to a heat treatment means using a conventional electric furnace (hereafter referred to as furnace annealing).
There are many types of laser light, but generally laser crystallization which uses laser light having a pulse emission type excimer laser as an emission source (hereafter referred to as excimer laser light) is employed. The excimer laser has the advantages of high output and being capable of repeated irradiation at a high frequency, and in addition, the excimer laser light has the advantage of having a high absorption coefficient with respect to a silicon film.
The problem drawing the most attention at present is how large can the grain size of a crystalline semiconductor film crystallized by laser light be made. Naturally, if one grain becomes large, then especially the number of grain boundaries crossing a channel forming region of a TFT will be reduced. It therefore becomes possible to improve the electric field effect mobility and the threshold voltage of the TFT, typical electrical characteristics.
Furthermore, relatively clean crystallinity is maintained within each grain, and in order to increase the TFT characteristics as stated above, it is preferable to form the TFT so as to have the channel forming region completely within one grain.
However, it is difficult to obtain a crystalline semiconductor film with a sufficiently large grain size by present techniques, and although there are reports of such films being obtained experimentally, at present this has not reached a level which can be put to practical use.
Experimental results such as those shown in Shimizu, K., Sugiura, O., and Matsumura, M., xe2x80x9cHigh-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Methodxe2x80x9d, IEEE Transactions on Electron Devices, Vol. 40, No. 1, pp. 112-7, 1993, have been obtained. A three layer structure of Si/SiO2/n+Si is formed on a substrate in the above publication, and is then irradiated by excimer laser light on both the Si layer side and the n+Si layer side. It is shown that a large grain size can be achieved by this type of structure.
The present invention has been made to solve the above problems, and an objet of the present invention is to provide a method of laser annealing for obtaining a crystalline semiconductor film having a large grain size, and to provide a laser apparatus which uses the laser annealing method. Further, another object of the present invention is to provide a semiconductor device, and a method of manufacturing the semiconductor device, using the laser annealing method.
The main point of the present invention resides in that laser light is irradiated on both the top surface of an amorphous semiconductor film (the surface on which thin films are formed) and the bottom surface of the amorphous semiconductor film (the surface opposite to the top surface) at the same time when crystallizing the amorphous semiconductor film, and that the effective energy strength of the laser light irradiated on the top surface (hereafter referred to as primary laser light) and the effective energy strength of the laser light irradiated on the bottom surface (hereafter referred to as secondary laser light) differ from each other.
That is to say, when the effective energy strength of the primary laser light is taken as (I0), and the effective energy strength of the secondary laser light is taken as (I0xe2x80x2), the laser light irradiated is characterized in that a relationship of 0 less than (I0xe2x80x2/I0) less than 1, or a relationship of 1 less than (I0xe2x80x2/I0) is formed for the ratio of effective energy strength(I0xe2x80x2/I0). Of course, I0xc2x7I0xe2x80x2xe2x89xa00.
Note that, throughout this specification, xe2x80x9ceffective energy strengthxe2x80x9d refers to the energy strength of the laser light when it reaches the top surface or the bottom surface of the -amorphous semiconductor film, and is defined as the energy strength after considering energy losses due to things such as reflection (the units are those of density, expressed as mJ/cm2). It is not possible to measure the effective energy strength, but provided that the media which exists along the laser light path is understood, the effective energy strength can be obtained by a calculation of the reflectivity and the transmittivity.
For example, a specific calculation method for effective energy strength is explained for the case of implementing the present invention in the structure shown in FIG. 6. In FIG. 6, reference numeral 601 denotes a aluminum reflecting body, reference numeral 602 denotes a Coming Co. #1737 substrate (thickness 0.7 mm), 603 denotes a 200 nm thick silicon oxynitride film (hereafter referred to as an SiON film), and 604 denotes a 55 nm thick amorphous silicon film. An example of irradiating XeCl excimer laser light with a wavelength of 308 nm on this type of test piece in the air is shown.
The energy strength of the laser light (wavelength 308 nm) just before arriving at the amorphous silicon film 604 is taken to be (Ia). At this point, the effective energy strength of the primary laser light (I0) is expressed as Io=Ia (1xe2x88x92RSi) in consideration of the laser light reflected on the surface of the amorphous silicon film. Note that RSi is the reflectivity of laser light. In this case, I0=0.45 Ia in the calculations.
Further, the effective energy strength of the secondary laser light (I0xe2x80x2) is expressed by I0xe2x80x2=Ia T1737RAlT1737 (1xe2x88x92RSiON-Si) where T1737 is the transmittivity of the #1737 substrate RAl is the reflectivity of the surface of the aluminum, and RSiON-Si is the reflectivity when the laser light is incident on the amorphous silicon film from within the SiON film. Note that the reflectivity of the laser light incident on the SiON film from within the air, the transmittivity within the SiON film, the reflectivity when incident on the #1737 substrate from within the SiON film, and the reflectivity when incident on the SiON film from within the #1737 substrate have been shown experimentally to be ignorable, and therefore they are not included in the calculations. In this case, I0xe2x80x2=0.13 Ia in the calculations.
Therefore, for the structure of FIG. 6, the effective energy strength of the primary laser light (I0) is found to be 0.45 Ia, and the effective energy strength of the secondary laser light (I0xe2x80x2) is found to be 0.13 Ia. In other words, the effective energy strength ratio (I0xe2x80x2/I0) is 0.29. One characteristic of the present invention is that the effective energy strength ratio determined as above satisfies the condition 0 less than (I0xe2x80x2/I0) less than 1.
Further, the present invention is even effective for cases in which the strength of the primary laser light is smaller than the strength of the secondary laser light. Namely, the present invention is also effective for cases in which the effective energy strength ratio satisfies the condition 1 less than (I0xe2x80x2/I0).
The following methods can be given for making the effective energy strengths of the primary laser light and the secondary laser light differ:
1) a method of attenuating the effective energy strength of the secondary laser light by regulating the reflectivity of the reflective body, and making it relatively smaller than the effective energy strength of the primary laser light, when irradiating the laser light on the top surface and the bottom surface of the amorphous semiconductor film by using the reflecting body formed underneath the substrate;
2) a method of forming the secondary laser light by partitioning the primary laser light along its path, effective energy strength of the secondary laser light using a filter (such as a variable attenuater), and making both effective energy strengths differ relatively;
3) a method of attenuating the effective energy strength of the secondary laser light by the substrate material on which the amorphous semiconductor film is formed, and making the effective energy strength of the secondary laser light relatively smaller than the effective energy strength of the primary laser light;
4) a method of sandwiching an insulating film between the substrate and the amorphous semiconductor film, damping the effective energy strength of the secondary laser light by the insulating film, and making it relatively smaller than the effective energy strength of the primary laser light;
5) a method of covering the top surface of the amorphous semiconductor film by an insulating film, making the reflectivity of the primary laser light on the top surface of the amorphous semiconductor film smaller, and making the effective energy strength of the primary laser light relatively larger than the effective energy strength of the secondary laser light;
6) a method of covering the amorphous semiconductor film by an insulating film, attenuating the effective energy strength of the primary laser light, and making it relatively smaller than the effective energy strength of the secondary laser light; and
7) a method of forming the primary laser light and the secondary laser light as separate laser emission sources respectively and differing both the effective energy strengths.
Furthermore, the present invention is not dependent upon the type of laser, and generally known lasers such as an excimer laser (typically a KrF laser or an XeCl laser), a solid state laser (typically an Nd:YAG laser or a ruby laser), a gas laser (typically an argon laser or a helium neon laser), a metal vapor laser (typically a copper vapor laser or a helium cadmium laser), and a semiconductor laser can be used.
Note that when using laser light having a fundamental wavelength which is long, such as the Nd:YAG laser (first harmonic: wavelength 1064 nm), it is preferable to use the second harmonic, the third harmonic, or the fourth harmonic. These harmonics can be obtained using non-linear shape crystals (non-linear shape elements). Further, a known q-switch method may also be used.
An explanation is offered, based upon the experimental results, regarding how the applicant of the present invention came up with the concept of the present invention. The SEM (scanning electron microscopy) photographs shown in FIGS. 7A and 7B are photographs of polysilicon films, formed by laser crystallization, on which Secco etching has been performed. Detailed information regarding the Secco etching technique can be found by referring to Secco d""Aragona, F., xe2x80x9cDislocation Etch for (100) Planes in Silicon,xe2x80x9d J. Electrochem. soc., Vol. 119, No. 7, pp. 948-950 (1972).
Each of the pieces was obtained by irradiating excimer laser light on an amorphous silicon film (film thickness 55 nm) formed on a Coming Co. #1737 substrate (substrate thickness 0.7 mm), through a silicon oxide film (film thickness 200 nm). Note that the excimer laser light used in this experiment was pulse laser light using XeCl gas as an excitation gas, having a wavelength of 308 nm, a pulse width of 30 ns, the number of shots set to 20 shots, and an energy density of 370 mJ/cm2.
FIG. 7A is a polysilicon film (average grain size approximately 0.3 xcexcm) obtained by irradiating the laser light on only the top surface of the amorphous silicon film, and FIG. 7B is a polysilicon film (average grain size approximately 1.5 xcexcm) obtained by irradiating the laser light on both the top surface and the bottom surface of the amorphous silicon film. This shows that the polysilicon film obtained by irradiating the laser light on both the top surface and the bottom surface of the amorphous silicon film has a grain size which is approximately 5 times larger, confirming that irradiation on both surfaces is extremely effective.
Note that the definition of average grain size used throughout this specification is based upon xe2x80x9cthe definition of grain region average sizexe2x80x9d used throughout the specification of Japanese Patent Application Laid-open No. Hei 11-219133.
It is thus confirmed that the grain size can be made larger by irradiating laser light on the top surface and the bottom surface of an amorphous semiconductor film. Note that experiments within the literature shown in the conventional examples do not irradiate direct laser light to the bottom surface of the semiconductor film being crystallized, and that an accumulated heat effect is aimed for by utilizing the residual heat of n+Si, a composition of which completely differs from the experiment performed by the applicant of the present invention.
Next, the applicant of the present invention performed a similar experiment using a quartz substrate as a substitute for the glass substrate (note that the laser light energy density was set to 200 mJ/cm2). The results obtained are as shown in FIGS. 8A and 8B (SEM photographs after Secco etching).
FIG. 8A is a polysilicon film obtained by irradiating laser light on only the top surface of an amorphous silicon film, and FIG. 8B is a polysilicon film obtained by irradiating laser light on both the top surface and the bottom surface of an amorphous silicon film. The figures show that, when using a quartz substrate as the substrate, the average grain size is from 0.4 to 0.5 xcexcm at best, and a large grain size such as that shown in FIG. 7B could not be found. Further, no difference was seen in grain size between irradiating from one surface of the substrate and irradiating from both sides of the substrate. In other words, as stated above, in spite of irradiating the laser light to both the top surface and the bottom surface of an amorphous semiconductor film, an effect that an average grain size was increased was not found.
The applicant of the present invention then took the above experimental results into consideration and conjectured that the difference in the experimental results shown in FIGS. 7A and 7B, and FIGS. 8A and 8B, is because of the difference between the transmittivity of the glass substrate (approximately 50%) and the transmittivity of the quartz substrate (approximately 93%), namely the difference between effective energy strengths of the laser light irradiated on the bottom surface of the amorphous semiconductor films. The following experiment was then performed for confirmation.
In this experiment, a test piece having the structure shown in FIG. 6 is first manufactured using a quartz substrate as a substrate 602 and using a tantalum nitride film as a reflecting body 601. XeCl excimer laser light was then irradiated on the test piece using conditions identical to those of the photograph obtained in FIG. 7B. The average grain size of the polysilicon film obtained was confirmed by a SEM photograph after Secco etching. These results are shown in FIG. 9.
As can be understood by looking at FIG. 9, the grain size of the polysilicon film obtained is distributed in a state which is nearly the same as that of the polysilicon film of FIG. 7B. Further, it has already been stated that the effective energy strength ratio between the primary laser light and the secondary laser light for the case of the test piece on which the photograph of FIG. 7B was obtained was 0.29. This is a result in which the secondary laser light is effectively attenuated by the glass substrate. A value of 0.33 was obtained when similarly calculating the effective energy strength ratio for the test piece of the present experiment. This is a result in which the secondary laser light is effectively attenuated by the reflecting body.
Furthermore, the test piece of FIG. 8B (a combination of reflecting bodies made from quartz and aluminum) and the test piece of FIG. 9 (a combination of reflecting bodies made from quartz and tantalum nitride) have the identical structure, except for the difference in the material on the surface of the reflecting body; the point of difference is that the reflectivity on the surface of the reflecting body on the test piece of FIG. 9 is smaller than that of the test piece of FIG. 8B.
Considering the above results, when the effective energy strength of the laser light (secondary laser light) on the bottom surface is smaller than the effective energy strength of the laser light (primary laser light) on the top surface for a case of crystallization by irradiating laser light on the top surface and the bottom surface of an amorphous semiconductor film, it has been confirmed that there is an increase in average grain size.